This invention generally relates to an interference filter having a glass substrate.
Substrates that demand high expansion with good chemical durability are often manufactured from optical glasses. Optical glasses may be employed in various applications, such as substrates for interference filters used in fiber optic systems. Generally, these interference filters are fabricated from multiple layers of conducting and insulating materials or films that together result in a filter that passes only a narrow bandwidth of incident radiation. Such filters are described in Optical Filter Design and Analysis-A Signal Processing Approach by Christie K. Madsen and Jian H. Zhao published by John Wiley and Sons, 1999.
The thermal expansion value of a glass substrate can have important implications on device performance. As an example, a mismatch in thermal expansion between the glass substrate and a coating film can impose undue stress on the film. This stress can be calculated by the following formula:
"sgr"=Efilmxcex94xcex1xcex94T
where Efilm is the Young""s modulus of the film, xcex94xcex1 is the mismatch in thermal expansion coefficient between the film and substrate, and xcex94T is the shift in temperature from the preparation temperature of the film to the temperature of use, which is usually room temperature.
One solution is to prepare and maintain the film at the temperature of its intended use. However, transient stresses develop even for slight departures from the film creation temperature. Therefore, it is highly desirable to minimize the mismatch in thermal expansion between the film and the glass substrate.
In one particular application, there is a strong demand for a glass substrate capable of being incorporated into an interference filter for dense wave division (DWD) or dense wave division multiplexing (DWDM) applications. Such interference filters have high requirements in narrowing the bandwidth of light transmission with minimal drift of this property with change in temperature. These filters require bandwidths of less than 200 GHz pass frequency in the 1.5 xcexcm wavelength region. Desirably, the substrate should be characterized by high transmission in the near IR where DWDM systems operate, i.e., wavelengths at or near 1.5 xcexcm, and a refractive index value at 587.6 nm, (nd) of 1.50 to 1.60. High transmission at 1.5 xcexcm is characterized by a value of digital transmittance exceeding 88%.
It is also desired that such substrate glasses not contain primary colorants such as Mn, Co, Ni, V, Fe, Cr, and Cu, e.g., because they can impede transmission or not contain rare earth additives such as Er oxide or Nd oxide, which can have absorption bands in the visible or near-infrared regions of the electromagnetic spectrum, or not include CoO, NiO2, FeO, Fe2O3 and CuO which can impede transmission in the near infrared region of the electromagnetic spectrum.
Furthermore, it would be desirable to manufacture an interference filter substrate economically. One such desirable process for making interference filter substrates from optical glasses is tank melting. In this case, refining agents such as arsenic oxide and antimony oxide are often used during the tank melting process to produce glass with high optical quality and low bubble content. However, often optical glass substrates contain cerium oxide. Cerium can form a solarization couple with either of these compounds, resulting in browning of the glass when it is exposed to short wavelength radiation. The glass browning can cause the loss in optical transmission. Consequently, glasses having cerium oxide would generally be undesirable for use as an interference filter substrate.
A desired embodiment of the invention is an interference filter for a fiber optic system including a substrate and a film coating the substrate. Typically, the substrate is coated with a series of layers of differing materials having properties, e.g., indices of refraction, producing interference effects achieving the desired wavelength transmission spectrum. Fiber optic systems comprise one or more light sources, fiber optic transmission components, a receiver of transmitted radiation, filters and end use components, e.g., detection, amplifiers, etc. In the prior art, it was desirable to have a glass substrate match the thermal expansion of a coating film. Surprisingly, it has been found that glass substrates having a thermal expansion much higher than that of the coatings applied to the substrate have significant and unexpected properties as a filter, such as having transmission characteristics independent of temperature. Generally, substrates having a coefficient of thermal expansion of about 130- about 210xc3x9710xe2x88x927/xc2x0 C.(xe2x88x9230xc2x0 C. to +70xc2x0 C.) are useful in this invention.